1. Field of the Invention
This invention relates to a high toughness silicon nitride sintered body suited for use in gas turbine blades, engine parts, etc. as well as to a process for producing the sintered body.
2. Description of the Prior Art
Heat-resistant alloys have heretofore been used in parts such as gas turbine blades and the like, exposed to high temperatures or other unfavorable environments. Recently, the use of such parts at high temperatures is in request for higher performances and accordingly the conditions under which heat-resistant alloys are used are approaching the use limit of such alloys. Hence, as a material replacing the heat-resistant alloys, a silicon nitrile sintered body excellent in heat resistance, oxidation resistance and thermal shock resistance has come to be paid attention.
However, because ceramics such as silicon nitrile and the like have, as well known, a large drawback of being brittle, they have not yet been put into practical application.
As a toughness-improved ceramic material, there are well known cermets disclosed in, for example, "CERAMIC BULLETIN", Vol. 60, No. 11 (1981), which are produced by subjecting a ceramic powder and a metal powder to pressure molding and then to sintering to obtain a heat-resistant material. In these ceramets, since the brittleness of ceramics is compensated by the presence of a metal phase, the higher strength and toughness at the normal temperature are obtained than that of ceramics only; however, owing to the presence of a metal phase, the upper limit of the heat resistance is low and accordingly the strength and hardness of the cermets are small at high temperatures. Therefore, although the cermets have a slightly improved toughness, the toughness has not yet reached a level enough for the cermets to be used as a high temperature structural material. Further, as seen in "JOURNAL OF MATERIALS SCIENCE", 7 (1972) 663-675, there is proposed the addition of a needle-like substance such as whiskers, fibers or the like to ceramics. It is known that, according to this proposal, even if cracks appear in ceramics, the direction of the cracks are turned owing to whiskers or the like dispersed in the ceramics and consequently the ceramics have an improved toughness. However, uniform dispersion of the needle-like substance in ceramics is very difficult. When the needle-like substance is, for example, whiskers or fibers, these whiskers or fibers get intertwined in the ceramics and tend to exist in the form of lumps which has prevented the development of tough ceramics, such as a silicon nitride sintered body having a strikingly improved toughness.
Ceramics obtained by adding to silicon carbide or nitride a boride, a carbide, a nitride, a silicide or the like of various metals exhibited, only in some cases, a slightly improved toughness. An attempt of adding a powder of a metal such as W, Mo or the like to silicon nitride, molding the resulting mixture and subjecting the resulting molding to high temperature sintering to convert the added metal component to a nitride did not provide a sintered body of high toughness which can be used practically.
Ceramics obtained by adding fine particles of a silicide or a carbide to silicon nitride, Si.sub.3 N.sub.4 is known. The present inventors have tried to improve the toughness of such ceramics.